At character creation, all abilities start at the minimum of 8 points, with 28 additional points to distribute between them to a maximum of 18 in any ability. Racial modifiers apply outside these limits, so it's possible, for example, for an Elf to have 6 Constitution and 20 Dexterity. After creation, all characters receive one point in the ability of their choice every fourth character level (4, 8, 12, etc...).

Strength (STR)
As a measure of raw physical power, Strength represents your character's athletic abilities. It's especially important for characters who focus on large, heavy-hitting melee weapons, like most Fighters, Barbarians, and Paladins. There are also certain obstacles in dungeons that will not be able to be passed without characters with extraordinary strength.

Effects
Melee Attack Bonuses
Melee Damage Bonuses
Thrown Weapon Damage Bonuses
Ranged Damage Penalties (if less than 10)
Jump, Swim
Feats of Strength
Special Conditions: Learning Weapon Finesse will allow a character to use their Dexterity bonus in place of their Strength bonus for Melee Attack rolls when using light weapons. Rangers automatically receive Bow Strength, which allows them to use their Strength bonus to increase ranged attack damage with a bow.

Racial Modifiers: Halflings get -2 Strength, but get a +1 attack bonus to make up for it.

Dexterity (DEX)
Quickness and agility are represented by a character's Dexterity. While it's obvious that Dexterity is critical for Rogues and Bards for their class-specific skills, any class that is going to be focusing on lighter armors, or no armor at all, will find Dexterity to be a life saver.

Effects
Reflex saves
Armor Class dodge bonuses
Ranged Attack bonuses
Thrown Weapon Attack Bonuses
Balance, Disable Device, Hide, Move Silently, Open Lock, Perform, Tumble
Special Conditions: The Armor Class bonus only applies if the character can react to the attack, so if you're attacked from behind, stunned, or suffering from any other similar affliction, you don't get the bonus. Rogues and Barbarians can lessen these penalties with their automatic Uncanny Dodge feats. Learning Weapon Finesse will allow a character to use their Dexterity bonus in place of their Strength bonus for Melee Attack rolls when using light weapons.

Racial Modifiers: Elves and Halflings both get +2 Dexterity, and Halflings also get an additional +1 Armor Bonus. Warforged, on the other hand, are always equipped with some form of armor plating that gives them a maximum dexterity bonus cap.

Constitution (CON)
Measuring your character health and stamina, Constitution grants bonus hit points at every level, making it incredibly important for anyone that is going to take damage. Which is just about everyone. Constitution also modifies Concentration checks for casters, making it doubly important for them, and your chances to resist poison and disease.

Effects
HP bonuses
Fortitude saves
Concentration
Racial Modifiers: Dwarves and Warforged both get +2 Constitution, and Warforged also have natural resistances to many physical afflictions. Elves and Halflings each get -2 Constitution, though Halflings get +1 on all saving throws.

Intelligence (INT)
Although it measures how well you learn and reason, Intelligence only has a major effect on the number of skills you learn per level. For certain skill-heavy classes, like Rogues, this is very important, but if your focus is on brawn you can afford to let Intelligence slip. Intelligence is absolutely critical for Wizards, who rely on it for all of their casting needs.

Effects
Skill points
Repair, Search
Wizard spells and spell points.
Wisdom (WIS)
Willpower and common sense are represented by Wisdom, which controls your chance to resist afflictions of the mind as well as your ability to notice things in the world around you. Like traps, secret doors, or a kobold rogue with a big rusty dagger. Wisdom is also critical for Clerics, Paladins, and Rangers, as it controls their spell casting abilities.

Effects
Will saves
Listen, Spot
Cleric, Paladin, and Ranger spells and spell points.
Racial Modifiers: Being artificial constructs, Warforged get -2 to Wisdom, but they are also naturally resistant to certain afflictions, like sleep.

Charisma (CHA)
Your force of personality is represented by Charisma, which includes such niceties as physical attractiveness, and modifies anything that involves others' opinions of you. This is absolutely critical for Sorcerers and Bards, whose spells and abilities are tied directly into their charm. Charisma also controls the turning abilities of Clerics and Paladins.

Effects
Bluff, Diplomacy, Haggle, Intimidate, Use Magic Device
Cleric and Paladin undead turning.
Sorcerer and Bard spells and spell points.